Just in case someone else has been stuck on the same issue, I did the following to setup the "Platinum" Group which I use as a membership site group:

- Added the Platinum Group as a User Group (It automatically became a secondary Group).
- At Applications -> Node Tree -> I clicked on "Permissions" next to the Platinum Group:.
- Then checked it as "private" and clicked on "Platinum Members", (not "Group Info") and to allowed access to the Platinum Group
by simply clicking on "Allow" next to "View Node".

The Platinum members can see this forum whereas the registered users cannot...